fix: Popup.Child parent hierarchy, ResourceDictionary resolution

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Controls/StyleFlowToPopup.xaml

@@ -0,0 +1,24 @@
+<Page
+    x:Class="Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls.StyleFlowToPopup"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:local="using:Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    mc:Ignorable="d"
+    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+
+	<Grid>
+		<Grid.Resources>
+			<Style TargetType="TextBlock">
+				<Setter Property="Foreground" Value="Red" />
+			</Style>
+		</Grid.Resources>
+		<StackPanel>
+			<TextBlock x:Name="InnerTextBlock" Text="Red foreground" />
+		</StackPanel>
+		<Popup x:Name="TestPopup">
+			<TextBlock x:Name="PopupContentTextBlock" Text="This should have red foreground too!" />
+		</Popup>
+	</Grid>
+</Page>

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Controls/StyleFlowToPopup.xaml.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Controls.Primitives;
+using Microsoft.UI.Xaml.Data;
+using Microsoft.UI.Xaml.Input;
+using Microsoft.UI.Xaml.Media;
+using Microsoft.UI.Xaml.Navigation;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+
+namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls;
+
+public sealed partial class StyleFlowToPopup : Page
+{
+	public StyleFlowToPopup()
+	{
+		this.InitializeComponent();
+	}
+
+	public void ShowPopup() => TestPopup.IsOpen = true;
+
+	public TextBlock GridTextBlock => InnerTextBlock;
+
+	public TextBlock PopupTextBlock => PopupContentTextBlock;
+}

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_Style.cs

@@ -1,17 +1,15 @@
 using System;
-using System.Collections.Generic;
-using System.Text;
 using System.Threading.Tasks;
-using Uno.UI.Extensions;
+using Microsoft.UI;
+using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Data;
 using Microsoft.UI.Xaml.Markup;
-using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Microsoft.UI.Xaml.Media;
 using Private.Infrastructure;
-using Microsoft.UI.Xaml;
-using Microsoft.UI.Xaml.Data;
-using Uno.UI.RuntimeTests.Helpers;
 using SamplesApp.UITests;
-using Microsoft.UI.Xaml.Media;
+using Uno.UI.RuntimeTests.Helpers;
+using Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml.Controls;
 
 namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml
 {
@@ -88,5 +86,24 @@ public async Task When_ImplicitStyle()
 
 			Assert.AreEqual(HorizontalAlignment.Left, cc.HorizontalContentAlignment);
 		}
+
+		[TestMethod]
+		[RunsOnUIThread]
+		public async Task When_Style_Flows_To_Popup()
+		{
+			var page = new StyleFlowToPopup();
+			TestServices.WindowHelper.WindowContent = page;
+			await UITestHelper.Load(page);
+
+			var foreground = (SolidColorBrush)page.GridTextBlock.Foreground;
+			Assert.AreEqual(Microsoft.UI.Colors.Red, foreground.Color);
+
+			page.ShowPopup();
+
+			await TestServices.WindowHelper.WaitFor(() => VisualTreeHelper.GetOpenPopupsForXamlRoot(TestServices.WindowHelper.XamlRoot).Count > 0);
+
+			var popupForeground = (SolidColorBrush)page.PopupTextBlock.Foreground;
+			Assert.AreEqual(Microsoft.UI.Colors.Red, popupForeground.Color);
+		}
 	}
 }

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls_Primitives/Given_Popup.cs

@@ -78,29 +78,10 @@ public void When_Closed_Immediately()
 		}
 
 		[TestMethod]
-		public async Task When_Child_Visual_Parents_Does_Not_Include_Popup()
+		public async Task When_Child_Visual_Parents_Do_Not_Include_Popup()
 		{
-			var popup = new Popup();
-			popup.XamlRoot = TestServices.WindowHelper.XamlRoot;
-			var button = new Button()
-			{
-				Content = "test"
-			};
-			popup.Child = button;
-			popup.IsOpen = true;
-			await WindowHelper.WaitForLoaded(button);
-			bool found = false;
-			DependencyObject current = popup.Child;
-			while (current != null)
-			{
-				if (current == popup)
-				{
-					found = true;
-					break;
-				}
-
-				current = VisualTreeHelper.GetParent(current);
-			}
+			var popup = await LoadAndOpenPopupWithButtonAsync();
+			bool found = SearchPopupChildAscendants(popup, element => element == popup, element => VisualTreeHelper.GetParent(element));
 
 			Assert.IsFalse(found);
 
@@ -110,6 +91,70 @@ public async Task When_Child_Visual_Parents_Does_Not_Include_Popup()
 
 		[TestMethod]
 		public async Task When_Child_Logical_Parents_Include_Popup()
+		{
+			var popup = await LoadAndOpenPopupWithButtonAsync();
+			bool found = SearchPopupChildAscendants(popup, element => element == popup, element => (element as FrameworkElement)?.Parent);
+
+			Assert.IsTrue(found);
+
+			// Should not throw
+			popup.IsOpen = false;
+		}
+
+		[TestMethod]
+		public async Task When_Child_Visual_Parent_Is_Canvas()
+		{
+			var popup = await LoadAndOpenPopupWithButtonAsync();
+			var child = (FrameworkElement)popup.Child;
+			var parent = VisualTreeHelper.GetParent(child);
+			Assert.IsInstanceOfType(parent, typeof(Canvas));
+#if HAS_UNO // It is actually a PopupRoot, but it is internal in WinUI
+			Assert.IsInstanceOfType(parent, typeof(PopupRoot));
+#endif
+
+			// Should not throw
+			popup.IsOpen = false;
+		}
+
+		[TestMethod]
+		public async Task When_Child_Logical_Parent_Is_Popup()
+		{
+			var popup = await LoadAndOpenPopupWithButtonAsync();
+			var child = (FrameworkElement)popup.Child;
+
+			Assert.AreEqual(popup, child.Parent);
+
+			// Should not throw
+			popup.IsOpen = false;
+		}
+
+#if HAS_UNO // PopupPanel is Uno-specific
+		[TestMethod]
+		public async Task When_Child_Visual_Parents_Do_Not_Include_PopupPanel()
+		{
+			var popup = await LoadAndOpenPopupWithButtonAsync();
+			bool found = SearchPopupChildAscendants(popup, element => element is PopupPanel, VisualTreeHelper.GetParent);
+
+			Assert.IsFalse(found);
+
+			// Should not throw
+			popup.IsOpen = false;
+		}
+
+		[TestMethod]
+		public async Task When_Child_Logical_Parents_Do_Not_Include_PopupPanel()
+		{
+			var popup = await LoadAndOpenPopupWithButtonAsync();
+			bool found = SearchPopupChildAscendants(popup, element => element is PopupPanel, element => (element as FrameworkElement)?.Parent);
+
+			Assert.IsFalse(found);
+
+			// Should not throw
+			popup.IsOpen = false;
+		}
+#endif
+
+		private async Task<Popup> LoadAndOpenPopupWithButtonAsync()
 		{
 			var popup = new Popup();
 			popup.XamlRoot = TestServices.WindowHelper.XamlRoot;
@@ -120,23 +165,23 @@ public async Task When_Child_Logical_Parents_Include_Popup()
 			popup.Child = button;
 			popup.IsOpen = true;
 			await WindowHelper.WaitForLoaded(button);
-			bool found = false;
-			DependencyObject current = button;
+			return popup;
+		}
+
+		private bool SearchPopupChildAscendants(Popup popup, Predicate<DependencyObject> predicate, Func<DependencyObject, DependencyObject> getParent)
+		{
+			DependencyObject current = popup.Child;
 			while (current != null)
 			{
-				if (current == popup)
+				if (predicate(current))
 				{
-					found = true;
-					break;
+					return true;
 				}
 
-				current = (current as FrameworkElement)?.Parent;
+				current = getParent(current);
 			}
 
-			Assert.IsTrue(found);
-
-			// Should not throw
-			popup.IsOpen = false;
+			return false;
 		}
 
 		[TestMethod]

src/Uno.UI/UI/Xaml/Controls/Popup/Popup.Base.cs

@@ -138,6 +138,15 @@ partial void OnIsOpenChangedPartial(bool oldIsOpen, bool newIsOpen)
 
 	partial void OnChildChangedPartial(UIElement oldChild, UIElement newChild)
 	{
+		if (oldChild is FrameworkElement oldChildFe && oldChildFe.LogicalParentOverride == this)
+		{
+			oldChildFe.SetLogicalParent(null);
+		}
+		if (newChild is FrameworkElement newChildFe)
+		{
+			newChildFe.SetLogicalParent(this);
+		}
+
 		if (oldChild is IDependencyObjectStoreProvider provider &&
 			provider.Store.ReadLocalValue(provider.Store.DataContextProperty) != DependencyProperty.UnsetValue)
 		{

src/Uno.UI/UI/Xaml/Controls/Popup/PopupPanel.cs

@@ -231,17 +231,13 @@ Popup.PlacementTarget is not null
 	private protected override void OnLoaded()
 	{
 		base.OnLoaded();
-		// Set Parent to the Popup, to obtain the same behavior as UWP that the Popup (and therefore the rest of the main visual tree)
-		// is reachable by scaling the combined Parent/GetVisualParent() hierarchy.
-		this.SetLogicalParent(Popup);
 
 		this.XamlRoot.Changed += XamlRootChanged;
 	}
 
 	private protected override void OnUnloaded()
 	{
 		base.OnUnloaded();
-		this.SetLogicalParent(null);
 
 		if (XamlRoot is { } xamlRoot)
 		{

src/Uno.UI/UI/Xaml/Controls/Popup/PopupRoot.cs

@@ -17,7 +17,7 @@
 
 namespace Microsoft.UI.Xaml.Controls.Primitives;
 
-internal partial class PopupRoot : Panel
+internal partial class PopupRoot : Canvas
 {
 	private readonly List<ManagedWeakReference> _openPopups = new();
 

src/Uno.UI/UI/Xaml/DependencyObjectStore.cs

@@ -19,6 +19,7 @@
 using Windows.ApplicationModel.Calls;
 using Microsoft.UI.Xaml.Controls;
 using System.Diagnostics.CodeAnalysis;
+using Microsoft.UI.Xaml.Media;
 
 
 
@@ -1637,9 +1638,13 @@ internal IEnumerable<ResourceDictionary> GetResourceDictionaries(
 					{
 						yield return fe.Resources;
 					}
-				}
 
-				candidate = candidate.GetParent() as DependencyObject;
+					candidate = fe.Parent as FrameworkElement;
+				}
+				else
+				{
+					candidate = VisualTreeHelper.GetParent(candidate) as DependencyObject;
+				}
 			}
 
 			if (includeAppResources && Application.Current != null)

src/Uno.UI/UI/Xaml/Media/VisualTreeHelper.cs

@@ -270,6 +270,12 @@ private static IReadOnlyList<Popup> GetOpenPopups(VisualTree visualTree)
 				return uiElement.GetVisualTreeParent() as DependencyObject;
 			}
 
+			if (realParent is PopupPanel)
+			{
+				// Skip the popup panel and go to PopupRoot instead.
+				realParent = GetParent(realParent);
+			}
+
 			return realParent;
 		}